1. Field of the Invention
The invention relates to a sample rate converting device, and more particularly, to a sample rate converting device which converts a sample rate of a discrete-time signal to another sample rate that is optional multiples of the original one.
2. Description of the Related Art
FIGS. 1A and 1B show typical first-order and second-order Delta-Sigma modulators. The Delta-Sigma modulator is basically for converting a signal into another signal with a smaller number of bits (e.g., a one-bit signal), such that both of them have almost the same frequency components in the low-frequency band, and the quantization noise of the latter is concentrated mostly in the high-frequency portion, as shown in FIGS. 2A and 2B.
FIG. 3 shows a typical over-sampled Delta-Sigma digital-to-analog converter. As shown in FIG. 3, an interpolator 100 expands an input signal and generates an over-sampled digital signal. The interpolator 100 includes a power-of-2 expander 101 and a digital low-pass filter 102. The over-sampled digital signal is converted, by a Delta-Sigma modulator 103, into a digital signal with a smaller number of bits in each sample (typically 1-bit). The digital signal generated by the Delta-Sigma converter 103 is further converted, by a DAC 104, into an analog signal. A high-frequency noise introduced by the Delta-Sigma converter 103 is filtered out by a low-pass filter (LPF) 105, and then an output analog signal is generated.
In the prior art, if two digital signals with different sample rates are to be mixed and converted into an analog signal, the typical method is to convert the two digital signals into analog signals and then mixed the two analog signals, as shown in FIG. 4A. However, the time period for the design and layout of the analog circuits is longer, and the property of the analog circuit tends to be influenced by the manufacturing processes. With the progress of the technology, the area of digital circuits is rapidly reduced. Therefore, more and more circuits are implemented in digital manners to reduce area, to shorten the design period, and to keep the design easily reused when the manufacturing processes are likely changed.
FIG. 4B shows how to mix two signals in a digital manner. In spite of some advantages over analog designs, some difficulties still exist. For example, if the expanding ratio of the interpolator is chosen to be power-of-2 (e.g., 64, 128 or 256), which may be easily implemented, the expanding ratio for the sample rate converter may not be power-of-2 or even not an integer, which will cost much more (e.g., computing units with a larger area or a higher operation speed). For example, in FIG. 4B, the sample rate f1=48 KHz, and the sample rate f2=50 KHz. If the expanding ratio for the interpolator is 128, the expanding ratio of the sample rate converter must be 128*48/50=122.88 so that the outputs have the same sample rate f=6.144 MHz. The conventional sample rate converter with non-integer converting ratio requires a lot of multiplications and will cost much more.